A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to emit a light image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”) while blocking the user's external view. These HMD displays are often referred to as virtual reality (“VR”) displays. Other HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD can serve as the hardware platform for realizing augmented reality (“AR”). With AR the viewer's image of the world is augmented with an overlaying CGI. Another term used to refer to various types of HMDs is a heads-up display (“HUD”). A HUD is any display that permits the user to view a CGI without having to look down or otherwise taking their eyes significantly off their head up forward position. Both VR and AR HMDs can be implemented as HUDs.
HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are currently limited due to the cost, size, weight, limited field of view, small eyebox, or poor efficiency of conventional optical systems used to implemented existing HMDs. Another drawback of conventional HMDs is that many are not well suited for individuals that wear prescription glasses.
FIG. 1 illustrates a conventional near-to-eye optical system 100 using a waveguide 105 with internal partially reflecting mirrors 110. In order to produce a useful image at eye 115, each incident angle of input light should correspond to a single output angle of emitted light. Since waveguide 105 guides light 120 from the input side to the output side with multiple internal reflections between the input and output sides, in order to preserve the one-to-one correspondence of input and output angles, this system uses collimated input light 120 of virtual images placed at infinity. For images placed closer than infinity (i.e., less than collimated light), waveguide 105 begins to create ghost images within the eyebox, which reduce the modulation transfer function (“MTF”) and image contrast and severely detracts from the user experience.
For most users an image placed at infinity is comfortable and easily focused on. However, for users that are near-sighted, an image placed at infinity is blurry and difficult, if not impossible, to bring into focus without the aid of a prescription lens. Thus, optical system 100 includes a refractive prescription lens 125, which increases the divergence of collimated light 120. The downside of optical system 100 is that refractive prescription lens 125 is often a bulky, heavy element and must be placed between waveguide 105 and eye 115. External placement of waveguide 105 on the outside of refractive prescription lens 125 can result in unusual looking eyewear that many users may be unwilling to wear and requires the same prescription be applied to light 120 as is applied to external ambient light 130.